The present invention generally relates to the field of diagnostic imaging and, more specifically, to an array of electroacoustic transducers and an electronic probe for three-dimensional imaging.
Transducer arrays are widely used for making ultrasound probes and they are the device for generating acoustic radiation beams or the device for receiving acoustic signals and for transforming them into electric signals. Generally the same transducer array is used alternatively both for generating acoustic radiation beams to be transmitted and for receiving acoustic pulses to be converted into electric signals. However the arrangement with an ultrasound probe provided with two transducer arrays operating independently one of which for transmitting acoustic radiation beams and the other one for receiving acoustic pulses cannot be ruled out.
With regards to conventional ultrasound probes, for example, the transmitting/receiving head comprises a front side from which acoustic radiation ultrasound beams are emitted in a direction of propagation towards a body under examination and on which front side pulses reflected by the body under examination impinge. The head has a back side opposite to the front side and it is oriented towards the inside of the casing of the ultrasound probe and towards means for supporting said head inside the casing.
Acoustic radiation beams are composed of acoustic pulses emitted by the individual transducer elements that are combined together such to generate an acoustic radiation beam having a predetermined direction of propagation and a predetermined focusing along said direction of propagation.
The transmitting/receiving head generally comprises, with an order starting from the back side towards the front side and corresponding to the direction of propagation of acoustic waves, a first layer composed of an array of contact electrodes, having each one a separate electric connection line to an electric contact pin being a part of a multi-pin electric connector and provided at one peripheral edge of the layer of contact electrodes. The layer composed of the array of contact electrodes is superimposed by a further layer composed of an array of piezoelectric elements. These can be composed of ceramic elements and they constitute the individual transducers converting electric excitation signals into acoustic pulses emitted from one surface thereof and/or converting acoustic pulses impinging thereon into electric signals. Each one of the transducer elements of the array is coincident with a contact electrode and is electrically connected thereto for example by means of a simple surface contact of each individual contact electrode of a corresponding transducer element. The array of contact electrodes and of the overlapping piezoelectric elements is supported by an acoustically and electrically insulating material that can be a simple supporting layer and/or can incorporate at least partially the electrodes and the piezoelectric elements filling at least for a portion of the thickness of the overlapping contact electrodes and piezoelectric elements the gaps there between. A third layer is composed of a ground electrode. It can be in the form of a continuous sheet overlapped to the side of the array of piezoelectric elements opposite to the one overlapped by the contact electrodes. As an alternative said third layer can be made like contact electrodes by an array of individual elements which are electrically separated one from the other and each one overlapping and being electrically connected only to one of the piezoelectric elements.
Generally on the third grounding layer there are provided one or more layers of acoustic matching layers acting for matching the acoustic impedance of the transducers to the acoustic impedance of the operation environment, for example of the body under examination in this case using the transmitting/receiving head into an ultrasound probe.
As it is clear from what disclosed above, within the transducer array, each transducer has a predetermined surface from which the acoustic pulse is emitted/received and transducers have a predetermined distance one with respect to the other, while the transducer array in turn has a predetermined length and width depending on the fact it is composed of only one row of transducers or of several rows of transducers.
Characteristics of transducer arrays depend on different dimensional elements both regarding the overall size of the transducer array and regarding the size of the individual transducer elements.
The equation defining the position of the natural focus within a transducer having a length D (defined as the aperture of the transducer) is the following:F=D2/4λ  (1)
where λ is the wavelength.
Therefore, the greater the aperture is, the deeper is arranged the natural focus. The need of making transducers with apertures as wide as possible arising there from.
If the number of transducers in a transducer array is determined (for example 192) and if the maximum operating depth of the transducer array is determined (for example 4-5 cm), so the formula (1) defines the pitch, i.e. the length of each transducer (for example 0.2 mm).
Each transducer element in turn has a radiation pattern which tends to diverge, with respect to the axis perpendicular to the surface emitting/receiving acoustic pulses (direction of propagation or incidence of the acoustic pulses), by an angle θ such that:Sin θ=0.6λ/a  (2)
where a denotes the radius of the transducer element (assuming it has a circular section).
With reference to the formula (2) it can be deduced that the larger the transducer element is, the less the radiation diverges and, therefore, the more the emitted radiation beam tends to be a tube with a diameter equal to the diameter of the transducer element. Vice versa, if the transducer element tends to approximate a point source so the emitted beam tends to become wider till theoretically taking a spherical radiation pattern.
Therefore if transducer elements of a transducer array emit in a very directional manner, that is they have a narrow radiation lobe in the direction orthogonal to the surface of the element emitting the acoustic radiation, so it is not easy or even it is impossible to combine individual acoustic pulses of the individual transducer elements such to achieve an overall acoustic pulse for the transducer array focused along a line of propagation which is deviated by a certain angle with respect to the line of propagation orthogonal to the emitting surface of the transducer array, i.e. it is impossible to steer the emitted acoustic radiation. Such action substantially is the same as electronically steered the beam such to bring the radiation lobe of each element to cover a direction offset by an angle with respect to the direction perpendicular to the surface emitting the acoustic radiation of each element. On the contrary if the radiation lobe is wide, spherical at a greatest extent, the emitted acoustic radiation has no preferential directions and so the emitted pulse can be used for any directions. Therefore, in addition to deviate the beam in any directions, all the transducer elements can give their contribution to the overall pulse focused on a predetermined focus point, thus increasing the focusing level and, therefore, improving the lateral resolution.
The above theory is described in more details in the following publication “Physics and Instrumentation of Diagnostic Medical Ultrasound” by Peter Fish, John Wiley & Sons, chapter 4, pages 27-49.
Obviously, if a linear probe is considered, wherein transducer elements are placed one near the other on only one row having a predetermined length, the transducer array will have an aperture corresponding to the length of said row. In such case transducer elements can have rectangular emitting surfaces with the shortest side parallel to the length of the row of adjacent transducer elements, the elements being arranged one near the other along the longest sides thereof and the longest sides of the transducer elements being provided as parallel to the width dimension of the row of transducers.
In this case, by reducing the size of the transducer elements in the direction parallel to the length of the row of the adjacent transducer elements, i.e. by making transducer elements more narrow, for each transducer element the acoustic field or the main lobe takes a cylindrical configuration and not a spherical configuration with the axis of the cylinder parallel to the length extension of the transducer element, that is in a direction parallel to the width dimension or of the longest side of the row of transducer elements. This can be applied also to probes with two-dimensional arrays comprising several rows of rectangular transducer elements such as described above.
Therefore in the present description and in the claims, when it is not expressly terminologically differentiated, the spherical or similar to spherical term is intended to also include a cylindrical or similar to cylindrical configuration, as a special condition wherein one of the angles is fixed expressed in polar variables.
In electronic probes for three-dimensional images, there is the need of having a wide surface of the transducer array forming the transmitting/receiving head in order to have a good beam focusing even at relatively deep depth.
A wide surface of the transducer array being desired, and at the same time the possibility of making high steering operations of the acoustic radiation beam being desired, the above theory provides the solution of a high number of very small transducer elements. However this requires a considerable increase of the number of lines connecting each transducer element to the unit generating the excitation electric signal and/or to the unit processing the electric signal corresponding to acoustic pulses received from each transducer element, obviously with major costs as well as with practical restrictions as regards the maximum number of transducer elements and so of channels due to the fact that each channel corresponds to a line that is constituted by a conductor of a multi-channel cable. In addition to the above there is obviously also a rise of costs and of the complexity of the hardware generating excitation pulses for transducer elements and processing reception signals of individual transducer elements.
Therefore considering the fact of keeping a restricted number of transducer elements in combination with an increase of the overall dimension of the transducer array, two contrasting conditions would arise: the fact of keeping radiant surfaces of the individual transducer elements small would lead to a focusing loss at deep depths, while it would lead to high beam steering, and the fact of increasing the radiant surfaces of the individual transducer elements would lead to lose the possibility of making high steering, but it would lead to a good focusing at deep depth.
Therefore it is currently not possible to make transducer arrays having a restricted number of transducer elements and preferably having a typical number of transducer elements for conventional ultrasound apparatus and at the same time allowing high quality three-dimensional echographies to be achieved. For carrying out three-dimensional echographies it is necessary to use transducer arrays having a great number of transducers, and therefore it is necessary to provide both probes and ultrasound apparatuses specifically intended for said function.
Therefore, there is a need for an improved array of electroacoustic transducers and an electronic probe for three-dimensional imaging.